Bonded structure of dissimilar metallic materials and method of joining dissimilar metallic materials

ABSTRACT

Disclosed herein are bonded structures and methods of forming the same. One embodiment of a bonded structure comprises first and second metallic layers and a bonding interface between the first and second metallic layers formed by diffusion and comprising a layer of at least one intermetallic compound. The intermetallic compound layer is formed in an area 52% or greater of an area of the bonding interface and has a thickness of 0.5 to 3.2 μm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Japanese Patent Application Ser.No. 2007-209742, filed Aug. 10, 2007, and Japanese Patent ApplicationSer. No. 2008-108663, filed Apr. 18, 2008, each of which is incorporatedherein in its entirety by reference.

TECHNICAL FIELD

The invention relates to bonding techniques for joining dissimilarmetallic materials such as steel and aluminum alloy, steel and titaniumalloy, and aluminum alloy and titanium alloy, at a bonding interfacebetween which materials is formed an intermetallic compound. The presentinvention also relates to the resulting structures.

BACKGROUND

Summary of Lectures at Japan Welding Society Meeting, 77th series, pages320 to 321, by Japan Welding Society, September 2005 discloses, forexample, that when joining dissimilar metals consisting of steel andaluminum, if silica and oxygen exist at suitable densities in anintermetallic compound reaction layer formed at a joining interface,excess growth of a reaction layer can be suppressed and joining strengthcan be increased. In particular, by using a steel sheet adjusted so thatan inner oxidation is 1.5 μm, a cross tensile strength of 1.4 kN(maximum) can be obtained by electric resistance spot welding and bycombining the steel sheet with an aluminum alloy sheet (A6022) 1.6 mmthick.

Summary of Lectures at Japan Welding Society Meeting, 78th series, pages162 to 163, by Japan Welding Society, April 2006 describes that at thetime of spot welding 980 MPa grade alloyed molten zinc-plated steelsheets 1.2 mm thick with an aluminum alloy sheet (A6022) 1.0 mm thick,two-step energization stimulates softening and melting of a platedlayer, whereby a wedge-shaped Al₃Fe₂ intermetallic compound is formed ina reaction interface layer resulting in a high cross tensile strength of1.2 kN.

BRIEF SUMMARY

Disclosed herein are bonded structures and methods of making the same.One embodiment of a bonded structure comprises a first metallic layer, asecond metallic layer overlying the first metallic layer, and a bondinginterface between the first and second metallic layers formed bydiffusion and comprising at least one intermetallic compound layer. Theintermetallic compound layer is formed in an area 52% or greater of anarea of the bonding interface and has a thickness of 0.5 to 3.2 μm.

A method of bonding dissimilar materials made from metals as disclosedherein comprises overlying a first metallic layer with a first meltingpoint and a second metallic layer with a second melting point to form abonding interface, wherein the first melting point is lower than thesecond melting point. The first and second metallic layers are rapidlyheated and subsequently rapidly cooled. A compound layer is formed bydiffusion comprising at least one intermetallic compound at the bondinginterface by heat treating the first and second metallic layers at aheat treatment temperature equal to or greater than a temperature atwhich dislocation loops and voids formed by atomic vacancies resultingfrom the rapid cooling are at least partially eliminated by a maincomponent metal of the first metallic layer.

Also disclosed herein is a method of increasing bond strength between adissimilar metal of an iron-based alloy and a dissimilar metal of analuminum-based alloy. The method comprises heat treating the dissimilarmetals bonded by rapid heating and rapid cooling, the heat treatingoccurring at a temperature ranging from 130° C. to 440° C. A compoundlayer is thereby formed by diffusion at a bond interface containing atleast one Fe—Al based intermetallic compound.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views, and wherein:

FIG. 1 is a graph showing a relation between a thickness and a crosstensile strength of a metallic compound layer formed at a bondinginterface of steel and aluminum alloy;

FIG. 2 is a schematic of a spot welding device used for joiningdissimilar metallic materials disclosed herein;

FIG. 3 is a plan view showing the shape of a test piece for crosstensile testing that is used for evaluation of a joining strength;

FIG. 4 is a graph showing a relation between an area ratio and a joiningstrength within a nugget of an intermetallic compound layer of thethickness ranging from 0.5 to 3.2 μm;

FIG. 5 is a graph showing a relation between a maximum value of acrystal grain size and a joining strength of an intermetallic compoundlayer;

FIG. 6 is a cross-sectional view showing a joining structure of azinc-plated steel sheet and an aluminum alloy, which is obtained asdisclosed herein;

FIG. 7A is a transmission electron microscopy photograph showing a stateof a joining structure before heat treatment of a first embodiment;

FIG. 7B is a transmission electron microscopy photograph showing a stateof a joining structure after heat treatment of a first embodiment;

FIG. 7C is an outward appearance photograph showing a fracture state ofa test piece by a cross tensile test of a first embodiment;

FIG. 8 is a transmission electron microscopy photograph showing ajoining structure obtained by a comparative example 1;

FIG. 9A is an outward appearance photograph showing a fracture state ofa test piece by a cross tensile test of a comparative example 2;

FIG. 9B is an outward appearance photograph showing a fracture state ofa test piece by a cross tensile test of a second embodiment;

FIG. 9C is an outward appearance photograph showing a fracture state ofa test piece by a cross tensile test of a third embodiment;

FIG. 9D is an outward appearance photograph showing a fracture state ofa test piece by a cross tensile test of a comparative example 3;

FIG. 10A is a schematic showing an outline of joining of dissimilarmetallic materials according to a fourth embodiment, together with aspot welding device;

FIG. 10B is a cross-sectional view showing a joining structure of azinc-plated steel sheet and an aluminum alloy, which is obtained by thefourth embodiment;

FIG. 10C is a transmission electron microscopy photograph showing ajoining structure obtained by the fourth embodiment; and

FIG. 11 is a transmission electron microscopy photograph showing ajoining structure obtained by a fifth embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The conventional methods discussed above for bonding or joining ofdissimilar metallic materials inevitably produce inefficient joiningconditions that are industrially unsuitable. If efficiency of conditionsis attempted, a thick compound layer containing an intermetalliccompound having a large grain size results without achieving thenecessary strength. In view of this, an effort has been made to joinmembers of dissimilar metals with high joining strength at theintermetallic layer at the joining interface in an efficient manner.

Hereinafter, a bonded structure of dissimilar metals and a method ofbonding dissimilar metals according to the invention will be describedfurther in detail and concretely with respect to certain embodiments.

FIG. 1 depicts the relationship between a thickness and a cross tensilestrength of a metallic compound layer formed at a joining interfacebetween a steel sheet of the thickness of 0.55 mm and a 6000 seriesaluminum alloy sheet. As used herein, the terms bonding and joining areinterchangeable. The sheets are joined together by using an alternatecurrent type spot welding device shown in FIG. 2 under various joiningconditions, such as electric current of 20000 to 30000 A, pressurizingforce of 150 to 600 kgf, and an energization time of 250 milliseconds orless. Thereafter, the sheets are heat treated under varied conditions,such as a treatment temperature of 140 to 500° C. and a treatment timeof 20 minutes to 7 hours. For FIG. 1, the data was obtained underconditions wherein 90% or more of the intermetallic layer was 1 μm orless in thickness so that the thickness of the intermetallic compoundlayer formed at the bonding interface is as uniform as possible afterwelding. The cross tensile test was performed by using a test pieceshaped and sized as shown in FIG. 3 and according to the methodprescribed in Japanese Industrial Standard JIS Z3137 (entitled Specimendimensions and procedure for cross tension testing resistance spot-andembossed projection-welded joints).

As apparent from FIG. 1, for obtaining a high strength bonded structureof dissimilar metals, it is necessary to control the thickness of ametallic compound layer formed at a constant area ratio by diffusionjoining. It will be understood that for attaining a bonding strength ofan aluminum alloy sheet material having a tensile strength of 210 MPa,i.e., a cross tensile strength of 0.6 kN or more, a control latitude ofthe thickness of the intermetallic compound layer should be set withinthe range from 0.5 to 3.2 μm. For attaining a cross tensile strength of0.9 kN or more, the control latitude should be set within the range from0.6 to 2.8 μm. For attaining a high strength of 1.2 kN or more, thethickness of the intermetallic compound layer should be set within therange from 0.8 to 2.5 μm. Namely, when the thickness of theintermetallic compound layer exceeds 3.2 μm, its contribution to thestrength is lowered, and a sufficient strength cannot be obtained whenthe thickness is less than 0.5 μm. Accordingly, the thickness of theintermetallic compound layer should be set within the range at leastfrom about 0.8 to about 2.5 μm.

In the joining nugget formed by a spot welding device, the intermetalliccompound is not always formed at the entire area of the nugget (joiningsurface), but a compound thickness distribution was seen within thenugget. The area ratio of the

intermetallic compound layer formed with the above-described thicknessand strength was obtained by observing the central cross section of thejoining portion, measuring the length by which the thickness of theintermetallic compound is within the above-described range, assumingthat donut-shaped joining zones are formed concentric with the nugget,and calculating, from the distance of the center of the nugget, theratio of the sum of the areas of the joining zones, to the nugget area(joining area). The result is shown in FIG. 4.

According to FIG. 4, if the ratio of the area at which the intermetalliccompound layer was formed so as to have the thickness in the range from0.8 to 2.5 μm to the joining area was 52% or more, a good joiningstrength was exhibited even at the lower limit of variations. When thearea ratio increased to 70% or more, the joining strength furtherincreased and a sufficient cross tensile strength of 0.9 kN or morecould be attained.

FIG. 5 shows the influence of a maximum crystal grain size as measuredalong the longitudinal diameter of an intermetallic compound on ajoining strength. It is seen that even if the thickness of theintermetallic compound layer becomes a little larger, the joiningstrength is increased if the crystal grain is small.

The rapid heating process and rapid cooling process introduces latticedefect and crystal restoration during the joining process. Thestructural variation of the crystal grain at the joining interface ofthe intermetallic compound was refined by the heat treatment subsequentto the joining process. For the crystal grain to grow or disappear,atoms need to move in the material, that is, diffusion is necessary. Thestructure formed by the rapid heating process and the rapid coolingprocess has a high system energy. For example, the rapid cooling processintroduces composition inclination, many voids and dislocation. Ametallic structure in such a high-energy condition is changed by heattreatment, reducing the structure to a low energy condition.Representative examples are phenomena such as uniformalization ofcomposition inclination, growth or disappearance of crystal grain,disappearance of dislocation loops and voids that are formed bygathering of atomic vacancies, an changing of crystal grain boundariesto low energy grain boundaries.

Under high temperature conditions, a number of atomic vacancies arecontained in a lattice, but if the metallic system is rapidly cooledfrom such a high temperature condition, vacancies exist in the cooledlattice with a high density. Such supersaturated vacancies increase thesystem energy. In case of heat treatment of a normal structure, sincethere exist vacancies in only the amount that is admittedthermodynamically, diffusion caused by the heat treatment is restrictedby the amount of vacancies and the speed of movement of vacancy. But byheat treatment of the structure having an excess amount of vacancies dueto rapid cooling, diffusion in excess of that caused by normal heattreatment can occur. To cause growth and disappearance of crystalgrains, it is necessary to elevate the temperature for a prolonged timeto attain a sufficient amount of vacancies and atomic movement. However,because a sufficient amount of vacancies exist in the crystal lattice,sufficient diffusion and reaction can occur even at a relatively lowtemperature.

Refinement of crystal grain, which is a feature of a structural changeof the embodiments disclosed herein, can occur under a heat treatmentcondition in which the number (frequency) of crystal nucleus formationsis high and the grain growth speed is not so high. The low-temperatureheat treatment of the rapidly cooled structure can increase the number(frequency) of nucleus formations due to the excess amount of vacanciesand can avoid, during the grain growth thereafter, coarsening of thecrystal grains.

For performing heat treatment under such a condition, high-temperatureheat treatment leading to coarsening of the crystal grain should beavoided, and the temperature sufficient to move the vacancies introducedby the rapid heating process and the rapid cooling process should be setto the lower limit value of the heat treatment temperature. Namely, forrestoration of the metallic structure, heat treatment should beperformed not at the temperature at which the electric resistance isrestored but at the temperature at which disappearance of the voids ordislocation loops formed by gathering of atomic vacancies occurs.Regarding crystal restoration, a detailed description is found in“Theory of Dislocation”, pp. 229-235, edited by Japan Metal Society andissued by Maruzen Co., Ltd. It can be said that, for disappearance ofvoids that are formed by gathering of atomic vacancies or distinction ofdislocation loops, heat treatment is performed at the temperature forthe “V step” or more of restoration of a metal structure.

For the crystal grain growth of the intermetallic compound in thediffusion joining interface between metallic materials of differentmelting points, sufficient diffusion of either base metal needs tooccur. Since the lower melting point base metal causes diffusion at thelower temperature, the grain growth of the intermetallic compound at theinterface is ruled by the diffusion of the lower melting point basemetal. For example, in the joining of iron and aluminum, refinement ofcrystal grains by heat treatment of a rapidly heated and rapidly cooledstructure, represented by electric resistance welding, starts to occurwhen the V step of restoration of an aluminum-base alloy is reached.This is because aluminum is a base material of a lower melting point ascompared with an iron-based alloy, i.e., at the temperature of 127° C.(400 K) or more. This temperature coincides with the temperature forcausing disappearance of dislocation loops and voids that are formed bygathering of the atomic vacancies of the rapidly cooled structure of thealuminum-base metal as the base metal. The structure caused by therapidly heating process and the rapidly cooling process changes into astable structure.

As having been described above, movement of the vacancies in the lowmelting point base metal lattice is a heat treatment condition of theembodiments herein. But for efficiently obtaining a target structurewithin a short time, it is desirable to perform heat treatment at thetemperature that causes sufficient diffusion. A sufficient diffusioneffect can be expected by setting the heat treatment temperature so asto be equal to or higher than ½ of the melting point expressed byabsolute temperature of the low melting point side base material.

On the other hand, when heat treatment is performed at the temperatureequal to or higher than ½ of the melting point expressed by absolutetemperature of the low melting point base material and at a highertemperature in case of a precipitation strengthened alloy for instance,the temperature at which the precipitation phase disappears is reached.Further, in case the strength is obtained by work hardening, thestrength of the base material is lowered by recrystallization. If theprecipitation phase, which strengthens the base material, disappears,the joining strength itself cannot be improved even if the strength ofthe joining portion is increased. Accordingly, the heat treatment afterjoining must allow the strength of the base material to be maintained atthe strength necessary for a structured body. Namely, heat treatment isperformed at the temperature equal to or lower than the temperature atwhich the low melting point base material is softened due todisappearance of the precipitation strengthening phase orrecrystallization.

Diffusion of the low melting point side base metal needs be sufficient,but if diffusion within intermetallic compound grains is excessive, itis possible that the strength cannot be attained due to the coarseningof the grains of the intermetallic compound. To avoid such coarsening ofthe grains of the intermetallic compound, it is necessary to performheat treatment at a temperature that causes diffusion within theintermetallic compound grains of the joining portion sufficiently, i.e.,that enables the vacancies to move easily within the intermetalliccompound. This temperature can be the temperature equal to or lower than½ of the melting point expressed by absolute temperature of theintermetallic compound whose melting point is lowest. A desiredtemperature range is one that causes movement of the vacancies withinthe low melting point side lattice and does not coarsen the metalliccompound grains, thereby attaining a sufficient strength of the basematerial. It has been discovered that it is industrially most effectiveto perform heat treatment at a highest possible temperature within thattemperature range. Namely, the heat treatment needs to be performed atthe temperature equal to or lower than one of: ½ of the melting pointexpressed by absolute temperature of the intermetallic compound whosemelting point is lowest among the formed intermetallic compounds and thetemperature at which the low melting point side metallic material issoftened by disappearance of precipitation strengthening phase orrecrystallization.

By energy dispersive x-ray (EDX) analysis of Al-Fe based intermetalliccompounds, a component ratio of the Al/Fe atomic ratio was observed tobe close to 3:1 or 5:2, and it was judged from this component ratio thatthe intermetallic compounds were Al₃Fe and Al₅Fe₂. The melting points ofthose intermetallic compounds are higher than the melting point ofaluminum, 660° C. The melting points of Al₃Fe and Al₅Fe₂ are 1160° C.and 1169° C., respectively. The aimed temperature for causing movementof the vacancies within the intermetallic compound structure can be setat ½ of the melting points expressed by absolute temperature, i.e.,about 440° C. To change the structure formed by the rapid heatingprocess and the rapid cooling process into a stable structure withoutcausing coarsening of the intermetallic compound, it is preferable toelevate the temperature equal to or higher than the temperature thateliminates the dislocation loops and voids that are formed by gatheringof the atomic vacancies of the rapidly cooled structure of the lowmelting point side base material, more preferably ½ of the melting pointexpressed by absolute temperature of the main component metal of the lowmelting point side metallic material. If heating is carried out at atemperature exceeding this temperature, coarsening of the intermetalliccompound grains was caused.

In diffusion joining, the joining strength becomes insufficient if theintermetallic compound is too thin. If the process temperature iselevated to achieve higher production efficiency, the resultingintermetallic compound grains are liable to be coarsened, possiblycausing a fragile characteristic. In diffusion joining, since thejoining is generally performed in a temperature range in which thediffusion speed is sufficiently large, the intermetallic compoundcrystal grains are liable to be coarsened. If it is tried to refine thecrystal grains, a long processing time is necessitated and animpracticable processing time is required for application to industrialproduction. In welding, since the temperature becomes high enough toreach the melting point, coarsening of the intermetallic compound occursin almost all cases.

As disclosed herein, by once causing electric resistance heating at thejoining interface, forming a structure of a composition in anunequilibrium state and of a uniform thickness and thereafter performingheat treatment at the temperature equal to or higher than thetemperature for causing disappearance of dislocation loops and voidsthat are formed by gathering of the atomic vacancies of the rapidlycooled structure of the low melting point metal, the intermetalliccompound layer having a uniform thickness at the time of joining ofdissimilar metallic materials is thereby obtained. In addition torefining the compound structure of an unequilibrium composition, astrong intermetallic compound layer of refined crystal grains andmoreover having a uniform thickness can be formed.

In embodiments herein, for dissimilar metallic materials, a combinationof Fe-based alloy and Al-based alloy can be used suitably. The Fe-basedalloy is herein intended to mean an alloy containing Fe as a maincomponent, and more specifically rolled steel of carbon steel, alloysteel, soft steel, high tensile steel or the like. Further, Al-basedalloy is intended to mean an alloy containing Al as a major component,and aluminum alloys from 1000 to 7000 series can be used suitably. Asused herein, “major component” is intended to indicate a metal that iscontained in the largest amount in an alloy. There is not any limitationto the combination of the dissimilar metallic materials used in theembodiments herein provided that the combination is such that thematerials contain, as main components, such metals that formintermetallic compounds at the interface by diffusion, wherein thediffusion is caused by a processing that utilizes a rapid heatingprocess and a subsequent rapid cooling process due to cooling of thematerials as electric resistant joining as described above. Examples ofa combination of such metal elements are a combination of Ti (titan) andAl (aluminum) and a combination of Ti and Fe (iron).

In the embodiments herein, if the dissimilar metallic materials includesa combination of Al-based alloy and Fe-based alloy, the above-describedheat treatment can be performed at the temperature equal to or higherthan the temperature that makes the main component metal of the lowmelting point side metallic material cause disappearance of dislocationloops and voids that are formed by gathering of the atomic vacancies ofthe rapidly cooled structure, i.e., 127° C. and in the temperature rangefrom about 130 to about 440° C., which temperature range is calculatedfrom ½ of the melting point expressed by absolute temperature of theintermetallic compound. Further, from the point of view of performingheat treatment in the temperature range equal to or lower than ½ of themelting point expressed by absolute temperature of the intermetalliccompound whose melting point is lowest amount the formed intermetalliccompounds, the temperature range equal to or higher than about 190° C.is desirable. Further, from the point of view of performing the heattreatment in the temperature range equal to or lower than one of: ½ ofthe melting point expressed by absolute temperature of the intermetalliccompound whose melting point is lowest among the formed intermetalliccompounds and the temperature at which the low melting point sidemetallic material is softened by disappearance of the precipitationstrengthening phase or recrystallization, the temperature range equal toor lower than about 410° C. is preferable.

A zinc-plated steel sheet can be used as the Fe-base alloy. By causingeutectic fusion between zinc forming a plating layer and aluminum andpressurizing the same, it becomes possible to remove the oxide coatingformed on the surface of the Al-based alloy together with the fusedeutectic material. It also becomes possible to attain diffusion joiningof new surfaces of both metallic materials from which the zinc-platinglayer and the oxide coating are removed.

In the method of joining dissimilar metals disclosed herein, for thejoining including a rapid heating process and a subsequent rapid coolingprocess, the electric resistance joining using a spot welding deviceshown in FIG. 2 can be representatively employed. However, this is anexample and the invention is not limited to the use such resistanceheating. Another heating means such as laser beam can be used. Further,the spot joining by nugget formation is also an example and is notlimiting, and, by using a roller-shaped electrode, seam joining can beperformed. In the method of joining dissimilar metals disclosed herein,if the joined member obtained is to be painted, the heat treatmentsubsequent to the above-described joining and baking of the paint in thepainting process can be performed at the same time. By this, the heatingprocess can be omitted, and energy can be conserved.

For a first embodiment, by using an alternate type spot welding deviceshown in FIG. 2, a Zn-plated steel sheet 2 of the thickness of 0.55 mmand a 6000 series aluminum alloy sheet 2 having a tensile strength of200 MPa are laid one upon the other and resistance spot joining thereofis executed under a condition of a pressurizing force of 300 kN, acurrent of 24000 A and an energizing time of 0.2 sec. At the time ofjoining, aluminum and zinc of the plating layer were once reacted at thetemperature of 400° C. to thereby cause eutectic fusion thereof. Anoxide coating 2 a on the surface of the aluminum alloy sheet 2 wasejected, as shown in FIG. 6, by electrode pressurization by the weldingdevice. The ejected oxide coating, as ejected matter D, together with afused eutectic material formed thereby cause a new surface of thealuminum alloy sheet 2 while causing diffusion reaction. Diffusionlayers were formed on the steel sheet 1 and the aluminum alloy sheet 2without melting the aluminum alloy. Fe and Al were reacted within thediffusion layers to form a thin intermetallic compound layer L, and thedissimilar metallic materials 1 and 2 were joined by way of theintermetallic compound layer L.

As a result, the aluminum oxide coating 2 a on the surface of thealuminum alloy sheet 2 is ejected together with Zn-Al reaction phase,i.e., an eutectic alloy, to the place around the nugget. The thinintermetallic compound layer L having the thickness in the range from0.8 to 2.5 μm is formed at the joining interface and in a region equalto 48% of the joining area (nugget area), and the intermetallic compoundcrystal grain of the intermetallic compound layer L is elliptical insectional shape, the long diameter being about 0.3 μm. A transmissionelectron microscopy photograph of the joining portion is shown in FIG.7A.

Then, the joined member prepared in this manner is subjected to heattreatment at 440° C. for 1.5 hours, and it was confirmed that the entireintermetallic layer came to have the thickness ranging from 0.8 to 2.5μm and was formed in the region equal to 89% of the joining area. At thesame time, the intermetallic compound grains were changed in an equiaxedmanner and the crystal grains could be refined so as to have a grainsize of 0.1 μm or less. A transmission electron microscopy photograph isshown in FIG. 7B. As a result of measurement of the strength of thejoined member having been heat treated at 440° C. for 1.5 hours by across tensile test, the outline of which is shown in FIG. 3, the joiningstrength of 1.60 kN was obtained, and it was confirmed that fracture wascaused at the aluminum alloy side as shown in FIG. 7C. In the meantime,as a result of investigation on the above-described intermetalliccompound layer L by EDS and X-ray diffraction analysis, it was confirmedthat the compound layer was constituted by intermetallic compoundshaving Fe—Al component ratios close to FeAl₃ and Fe₅Al₂, and Zinc of theplating layer was so scarcely contained in the intermetallic compound asnot to be found by the analysis.

For comparative example 1, the joined member was obtained under the samecondition as the first embodiment. The joined member was heat treated at500° C. for 0.5 hours. The intermetallic compound layer came to have thethickness exceeding 3.2 μm, the area ratio thereof was 98%, and thediffusion joining layer had the interface having the intermetalliccompound layer. However, there was scarcely any joining layer in whichthe thickness of the intermetallic compound layer was 3.2 μm or less andthe joining area was 2%. Further, it was observed that the crystalgrains were changed to constitute two layers, an equiaxed portion and apost-like portion. The crystal grains at the equiaxed portion were inthe range from 0.1 to 0.2 μm, and the crystal grains at the post-likeportion extended vertically so as to have a maximum long diameter closeto 1.0 μm. A transmission electron microscopy photograph in this case isshown in FIG. 8. In the cross tensile strength, fracture was caused atthe joining interface and the strength obtained was only about 0.09 kN.

For comparative example 2, the same spot welding device as theabove-described first embodiment was used. After a Zn-plated steel sheet1 of the thickness of 0.55 mm and a 6000 series aluminum alloy sheet 2of the thickness of 1.0 mm were laid one upon the other, resistance spotjoining was performed under a condition of a pressuring force of 300 kN,current being made smaller than that of the first embodiment and of20000 A and an energization time of 0.2 sec. for thereby joining thedissimilar metallic materials by diffusion joining. As a result, anintermetallic compound layer L was formed and the region of thethickness in the range from 0.8 to 2.5 μm was 46% of the joining area.Further, the maximum long diameter of the crystal grains of theintermetallic compound constituting the intermetallic compound layer Lwas 0.06 μm.

FIG. 9A shows a fracture state of a test piece in case the cross tensiletest was executed without processing the joined member obtained in themanner as described above by heat treatment. Although the nugget partlyremained on the steel sheet side, it was mostly fractured and peeled offfrom the joining interface, and the strength of 0.67 kN was obtained.

A second embodiment was a result of heat treating the joined memberprepared under the same conditions as the comparative example 2, at 440°C. for 1.5 hours. It was confirmed that the area of the intermetalliccompound layer L having the thickness in the range from 0.8 to 2.5 μmwas enlarged to 90% of the joining area, and the crystal grains of theintermetallic compound were changed in an equiaxed manner and refined sothat the grain size was 0.1 μm. FIG. 9B shows a fracture state of a testpiece where the cross tensile test was executed after theabove-described heat treatment. Plug fracture was caused, and the crosstensile strength reached to 1.69 kN. In this manner, a fracture strengththat was relatively high as compared with the base material strength wasobtained, though there occurred a change to a plug fracture mode.Without being bound to theory, this is considered due to an influence ofan increase of the base material strength, which was caused by aging ofthe aluminum alloy. In contrast to fracture being a peel off type beforeheat treatment, fracture was not started at the joining interface but atthe base material, signifying an increase of the strength of the joininginterface.

A third embodiment of the joined member was prepared under the samecondition as the above-described comparative example 2 but was heattreated at 300° C. for 7 hours. As a result, the region of theintermetallic compound layer L having the thickness in the range from0.8 to 2.5 μm was enlarged to account for 82% of the joining area.Regarding the crystal grains of the intermetallic compound, it wasconfirmed that the crystal grains changed in an equiaxed manner and wererefined so as to have the grain size in the range from 0.05 to 0.1 μm.FIG. 9C shows a fracture state of a test piece where the cross tensiletest was executed after the above-described heat treatment. The crosstensile strength was 1.50 kN, and it was possible to cause nuggetfracture.

Comparative example 3 was a result of heat treating a joined memberprepared under the same condition as the above-described comparativeexample 2, but at 500° C. for one hour. It was observed that theintermetallic compound layer L grew so as to have a thickness exceeding3.2 μm, and the area thereof was enlarged to almost 100% of the joiningarea. The crystal grains of the intermetallic compound were changed toconstitute two layers, an equiaxed portion and a post-like portion. Thegrain size was in the range from 0.1 to 0.3 μm at the equiaxed portion,and the maximum long diameter of the crystal grains at the post-likeportion was 1.8 μm. FIG. 9D shows a fracture state of a test piece incase the cross tensile test was executed after the joined member washeat treated at 500° C. for one hour. Interface fracture was caused, andthe cross tensile test result was only 0.08 kN. In this manner, asufficient strength can be realized by controlling the thickness andarea of the intermetallic compound layer to within a predeterminedrange. Where a sufficiently thin intermetallic compound layer is formedat a joining process prior to heat treatment, the strength can beincreased by making, by subsequent heat treatment, the thickness of theintermetallic compound increase within the limits that enable refinementof the grain size of the intermetallic compound. Where, to the contrary,the intermetallic compound layer formed at the joining step becomesrelatively thick, the quality of the joining portion of dissimilarmetals can be stabilized by refining the crystal grains while selectinga heat treatment condition that does not cause the intermetalliccompound layer to increase in thickness. In this manner, by a process offorming a rapidly heated and cooled structure, a typical example ofwhich is electric resistant joining, and a subsequent heat treatmentprocess, a joining portion of dissimilar metals having a high qualitycan be obtained stably. In addition, by combining thereto an agingcondition or the like, a joining of dissimilar metals can result inhigher strength.

As a fourth embodiment, shown in FIG. 10A, a molten zinc plating steelsheet 1 of the thickness of 0.55 mm and a 6000 series aluminum alloysheet 2 having the tensile strength of 210 MPa and the thickness of 10mm was laid one upon the other by way of a thermo-hardening adhesiveagent S, and resistant spot joining thereof was performed under thecondition of a pressuring force of 300 kN, a current of 24000 A and anenergizing time of 0.2 sec. to thereby join dissimilar metallicmaterials by diffusion joining without melting the aluminum alloy sheet2. At this time, aluminum and zinc of the plating layer were oncereacted at the temperature of 400° C. to thereby cause eutectic fusionthereof. An oxide coating 2 a of the aluminum ally sheet 2 and theadhesive agent were ejected by electrode pressurization by the weldingdevice as ejected matter, which together with a fused eutectic materialformed, thereby caused a new surface of the aluminum alloy sheet 2.Diffusion layers were formed on the steel sheet 1 and the aluminum alloysheet 2. An intermetallic compound layer L was formed at the joininginterface, and, as shown in FIG. 10B, the dissimilar metallic materials1 and 2 were joined by way of the intermetallic compound layer L withoutmelting the aluminum alloy.

Thereafter, by performing heat treatment at 170° C., which is thehardening temperature of the above-described adhesive agent S, theintermetallic compound L was formed in the region of 56% of the joiningarea so as to have a thickness in the range from 0.8 to 2.5 μm. It wasobserved that the crystal grains of the intermetallic compound wererefined so that a maximum long diameter was in the range from 0.05 to1.0 μm (refer to FIG. 10C). As a result of performing the cross tensiletest similarly, a joining strength of 0.94 kN was obtained.

In this embodiment, by heat treatment at one time, the adhesive agent Scan be hardened simultaneously with refinement of the intermetalliccompound layer to thereby form an insulation layer between thedissimilar metallic materials, thus making it possible to improve thejoining strength and the corrosion resistant ability without increasingthe number of process steps and the amount of invested energy. In themeantime, it is needless to say that such a technique can similarly beapplied to the case the member is painted after joining

To make a fifth embodiment, at the time of joining dissimilar metalsconsisting of a steel sheet 1 and an aluminum alloy sheet 2, thealternate type spot welding device shown in FIG. 2 was used, and thesame operations as the first embodiment were repeated except that azinc-plating layer 2 b was applied onto the surface of the aluminumalloy sheet 2. The joined member of dissimilar metals of the fifthembodiment is shown in FIG. 11. As a result, an intermetallic compoundlayer L having the thickness in the range from 0.5 to 3.5 μm was formedat the joining interface, and the region of the intermetallic compoundlayer having the thickness in the range from 0.8 to 2.5 μm was 56% ofthe joining area. It was found that the maximum long diameter of theintermetallic compound layer grains was 0.05 μm. Further, as a result ofperforming the cross tensile test, the joining strength of 1.2 kN wasobtained

Generally, a strong, high-melting point oxide coating exists on thesurface of the aluminum alloy 2 and its removal is a problem at the timeof diffusion joining But since in this embodiment the oxide coating hasbeen removed at the process of applying a zinc-plating to the aluminumalloy sheet 2 and a new surface of the aluminum alloy sheet 2 is exposedby melt removal, diffusion joining of the aluminum alloy sheet with thesteel sheet by energization heating is possible.

The above-described embodiments have been described in order to alloweasy understanding of the invention and do not limit the invention. Onthe contrary, the invention is intended to cover various modificationsand equivalent arrangements included within the scope of the appendedclaims, which scope is to be accorded the broadest interpretation so asto encompass all such modifications and equivalent structure as ispermitted under the law.

1. A bonded structure of dissimilar materials made from metalscomprising: a first metallic sheet; a second metallic sheet overlyingthe first metallic sheet; and a bonding interface between the first andsecond metallic sheets formed by diffusion and comprising at least oneintermetallic compound layer, wherein the intermetallic compound layeris formed in an area 52% or greater of an area of the bonding interfaceand has a thickness of 0.5 to 3.2 μm.
 2. The bonded structure accordingto claim 1 wherein the intermetallic compound layer is formed in an area70% or greater of the area of the bonding interface.
 3. The bondedstructure according to claim 1 wherein the intermetallic compound layerhas a thickness of 0.6 to 2.8 μm.
 4. The bonded structure according toclaim 1 wherein the intermetallic compound layer has a thickness of 0.8to 2.5 μm.
 5. The bonded structure according to claim 1 wherein theintermetallic compound layer is comprised of a crystal grain with alongitudinal diameter of 1.0 μm or less.
 6. The bonded structureaccording to claim 1 wherein one of the first metallic sheet and thesecond metallic sheet comprises an iron-based alloy and the othercomprises an aluminum-based alloy.
 7. The bonded structure according toclaim 6 wherein the intermetallic compound comprises an Fe—Al basedcompound.
 8. A method of bonding dissimilar materials made from metalscomprising: overlying a first metallic sheet with a first melting pointand a second metallic sheet with a second melting point to form abonding interface, wherein the first melting point is lower than thesecond melting point; rapidly heating the first and second metallicsheets; rapidly cooling the first and second metallic sheets; andforming a compound layer by diffusion comprising at least oneintermetallic compound at the bonding interface by heat treating thefirst and second metallic sheets at a heat treatment temperature equalto or greater than a temperature at which dislocation loops and voidsformed by atomic vacancies resulting from the rapid cooling are at leastpartially eliminated by a main component metal of the first metallicsheet.
 9. The method of claim 8 wherein the heat treatment temperatureis equal to or lower than ½ of a melting point in absolute temperatureof the at least one intermetallic compound whose melting point is lowestamong the at least one intermetallic compounds.
 10. The method of claim8 wherein the heat treatment temperature is equal to or higher than ½ ofa melting point in absolute temperature of a main component metal of thefirst metallic sheet.
 11. The method of claim 8 wherein the heattreatment temperature is equal to or lower than one of: ½ of a meltingpoint in absolute temperature of the at least one intermetallic compoundwhose melting point is lowest among the at least one intermetalliccompounds, and a temperature at which the first metallic sheet issoftened by disappearance of a precipitation strengthening phase orrecrystallization.
 12. The method of claim 8, wherein the rapid heatingand the subsequent rapid cooling occurs by electric resistance joiningby energization heating.
 13. The method of claim 8, wherein the rapidheating and the subsequent rapid cooling occurs simultaneously withbaking in a painting process.
 14. A method of increasing bond strengthbetween a dissimilar metal of an iron-based alloy and a dissimilar metalof an aluminum-based alloy, the method comprising: heat treating thedissimilar metals bonded by rapid heating and rapid cooling, the heattreating occurring at a temperature ranging from 130° C. to 440° C.,thereby forming a compound layer by diffusion at a bond interfacecontaining at least one Fe-Al based intermetallic compound.
 15. Themethod of claim 14 wherein heat treating occurs at a temperature equalto or higher than 190° C.
 16. The method of claim 14 wherein heattreating occurs at a temperature equal to or lower than 410° C.
 17. Themethod of claim 14 wherein the iron-based alloy is a zinc-plated steelsheet and wherein, prior to heat treating, the method further comprises:eutecticly fusing zinc of the zinc-plated steel sheet and aluminum ofthe aluminum-based alloy, thereby forming a low melting point fusedmaterial; removing an oxide coating on a surface of the aluminum-basedalloy at the bonded interface together with the fused material; andbonding by rapid heating and rapid cooling new surfaces of bothdissimilar metals from which a zinc-plating layer and the oxide coatingare removed.
 18. The method of claim 17 wherein the rapid heating andthe subsequent rapid cooling occurs by electric resistance joining byenergization heating.
 19. The method of claim 17 wherein the rapidheating and the subsequent rapid cooling occurs simultaneously withbaking in a painting process.